The initial rate of absorption of brick
When brick and mortar are brought together, a tug of war begins between the suction of the brick (trying to draw water out of the mortar) and the water retention qualities of the mortar (trying to keep its water to itself). Water retention is a manufactured quality and can vary between manufacturers and types of mortar. The objective is to keep the water and cement together, so they can be absorbed as a paste into the brick, and let the brick absorb as much of the paste as possible. This makes a good extent of bond—whether it is Portland cement/lime mortar, masonry cement, or mortar cement.
The suction properties of brick, measured as the initial rate of absorption (IRA), produce the bond between brick and mortar when a certain amount of cement/lime paste is absorbed into the pores of the brick. However, if the brick is too dry the brick suction prematurely draws too much water from the mortar, leaving a dry paste behind. If it is too wet, the brick has too little suction—reducing the extent of bond and weakening compressive strength. An experienced mason can feel if the mortar is not sticking properly to the brick because of mismatched moisture properties and adjust by rewetting brick or retempering mortar which has gotten too dry on the board.
Workmanship
Mortar droppings in the cavity can cling to the cross member of the reinforcement and absorb water. The mason can reduce mortar dropping into the cavity by sloping the bed joint mortar somewhat away from the cavity. Moisture that penetrates the veneer brick does so through the head joint; Fully buttering the head joint will reduce this defect. Additionally, one must not slush mortar on the joints. CMU mortar joints on the cavity side of the CMU must be cut flush to allow rigid insulation to fit tight to the wall and prevent creating a hollow space where moisture can collect.
Conclusion
When masonry walls encounter problems, water-related issues are often one of the primary factors. Brick masonry exposed to a disproportionate amount of water may have dimensional changes; efflorescence on exterior surfaces; and cracking, crazing, spalling, or disintegration due to repeated freeze-thaw cycling. Water may also cause metals to corrode, insulation to lose its effectiveness, and interior finishes to deteriorate.
Author’s note: This article is a presentation of the author’s experience and research and does not constitute legal or design advice. If the reader is contemplating construction, it is their responsibility to hire architects and engineers to design the work, prepare construction documents, and give the owner professional design advice during construction.
Notes
1Read The Cavity Wall: A Brief History, published by the London Amp Company. londondampcompany.co.uk/the-cavity-wall-a-brief-history.
2In the early 1960s research was conducted in Norway on rain penetration of windows and walls, and Øivind Birkeland published a treatise referring to a “rain barrier.” In 1963 the Canadian National Research Council published a pamphlet titled “Rain Penetration and its Control” using the term “open rain screen.”
Paul Potts is a technical writer, owner’s representative, and construction administrator. He has worked in the construction industry as an independent contractor and administrator for architects, engineers, and owners in Michigan since 1980. Potts can be contacted at paulpotts1@comcast.net.
